docs: def combine(self, pubkeys):
          """Add a number of public keys together."""
          assert len(pubkeys) > 0
  
          outpub = ffi.new('secp256k1_pubkey *')
          for item in pubkeys:
              assert ffi.typeof(item) is ffi.typeof('secp256k1_pubkey *')
  
          res = lib.secp256k1_ec_pubkey_combine(
              self.ctx, outpub, pubkeys, len(pubkeys))
          if not res:
              raise Exception('failed to combine public keys')
  
          self.public_key = outpub
          return outpub